In the semiconductor fabrication process, circuit structures are commonly formed on a substrate using a photolithography process. Photolithography involves coating a photoresist layer on a substrate, forming a device pattern in the photoresist and developing the photoresist. The photolithography process is followed by forming a metal conductive layer on the substrate according to the circuit pattern defined by the developed and patterned photoresist.
In a conventional photolithography process, the photoresist is coated on the surface of the substrate. A baking process is then used to evaporate solvent in the photoresist and densify the resist. This is followed by an exposure process in which light is transmitted through a patterned mask onto the photoresist. The exposed photoresist is subjected to cleavage, followed by photoresist development. At the development step, the photoresist is exposed to a liquid developer solution such as TMAH. The developer solution removes the unexposed polymer portions of the photoresist from the substrate. The developed photoresist is then subjected to water rinsing typically using DI water. Finally, a post-rinse spin drying step is carried out. An additional baking step may be used to evaporate any additional moisture from the photoresist surface.
A photoresist structure 10 is shown in FIGS. 1A and 1B and includes a BARC (bottom anti-reflective coating) layer 14 provided on a substrate 12. Adjacent photoresist lines 16 of a developed photoresist layer are provided on the BARC layer 14. A space 18 is defined between adjacent photoresist lines 16. The photoresist lines 16 define the boundaries of the circuit pattern which is to be subsequently formed on the substrate 12 after photolithography.
After development of the photoresist, open hydrophobic areas 20 frequently exist on the surface of the BARC layer 14 where exposed polymer portions of the positive photoresist were removed from the substrate. The lateral surfaces of the developed photoresist lines 16 are hydrophilic. The hydrophilic surfaces of the lateral surface of the lines 16 and the hydrophobic surface of the BARC layer 14 induce watermarks, which is near the line pattern, on the surface of the BARC layer 14 after the developed photoresist is rinsed and then subjected to the spin-drying step. Furthermore, some of the water from the post-development rinsing step is trapped in the space 18 between the photoresist lines 16. During the spin-drying and post-development baking steps, the trapped water shrinks and induces pattern collapse. The resulting capillary action between the photoresist lines pulls the water from between the lines to the open hydrophobic areas. Consequently, the photoresist lines 16 collapse toward each other, as shown in FIG. 1B. This compromises the structural integrity of the circuit pattern which is subsequently formed on the substrate 12. The water inbetween the two adjacent lines and on the BARC surface also becomes a watermark defect after developing (FIG. 2B).
Therefore, a method of inhibiting photoresist pattern collapse and defects during post-development photoresist processing steps is needed.